docs: def fill_subparser(subparser):
      """Sets up a subparser to convert YouTube audio files.
  
      Adds the compulsory `--youtube-id` flag as well as the optional
      `sample` and `channels` flags.
  
      Parameters
      ----------
      subparser : :class:`argparse.ArgumentParser`
          Subparser handling the `youtube_audio` command.
  
      """
      subparser.add_argument(
          '--youtube-id', type=str, required=True,
          help=("The YouTube ID of the video from which to extract audio, "
                "usually an 11-character string.")
      )
      subparser.add_argument(
          '--channels', type=int, default=1,
          help=("The number of audio channels to convert to. The default of 1"
                "means audio is converted to mono.")
      )
      subparser.add_argument(
          '--sample', type=int, default=16000,
          help=("The sampling rate in Hz. The default of 16000 is "
                "significantly downsampled compared to normal WAVE files; "
                "pass 44100 for the usual sampling rate.")
      )
      return convert_youtube_audio